Interest in the field of adaptive optics has increased in recent years due to the need for higher quality optical systems. In particular, higher quality deformable mirrors are required for applications such as photolithography, optical data storage, high power lasers, and atmospheric compensation.
A conventional deformable mirror includes a top plate which forms a reflective surface. The top plate is supported by an array of piezoelectric or electrorestrictive actuators which sit on a thick base for support. In operation, the actuators are elongated due to an electric field, causing the top plate to be deformed in a controlled fashion. This conventional deformable mirror design has been used for many years, but the mirrors are very costly because of all the hand assembly required in their fabrication.
The fabrication limitations of conventional deformable mirrors and the need for inexpensive adaptive optics systems inspired several attempts to produce micromachined deformable mirrors. For example, Vdovin et al. disclose a micromachined deformable mirror in an article entitled "Flexible Mirror Micromachined in Silicon", Applied Optics, Vol. 34, No. 16, June, 1995. The mirror includes a silicon nitride membrane which forms the mirror surface. The nitride membrane is mounted above aluminum electrodes. In operation, the electrodes electrostatically deform the membrane. Unfortunately, the use of a silicon nitride membrane in a deformable mirror has two disadvantages. First, the thick layer of nitride required for the membrane is costly. Second, the silicon nitride membrane has low reflectivity, low power handling capability, and poor adjacent channel crosstalk.
Another deformable mirror design is disclosed by Bifano et al. in "Continuous-Membrane Surface-Micromachined Silicon Deformable Mirror", Optical Engineering, Vol. 36, No. 5, p.1354-60, May, 1997. Bifano discloses a deformable mirror produced by surface micromachining three layers of polycrystalline silicon and two sacrificial layers of silicon dioxide which separate the layers of polysilicon. The top layer of polysilicon forms the mirror surface. The bottom layer of polysilicon is used to create an array of electrode pads. The middle layer of polysilicon is patterned into an array of fixed-end double cantilevers which act as second electrodes for deforming the mirror. After the polysilicon layers are patterned, the sacrificial oxide layers are removed by drilling holes in the mirror surface and etching the mirror with hydrofluoric acid.
Bifano's mirror design suffers from several disadvantages. First, the top layer of polysilicon which forms the mirror surface is not polished to optical quality. Second, because the mirror surface must be drilled and etched in hydrofluoric acid, the mirror exhibits low reflectivity and high scatter loss. Thus, the micromachined mirrors presented thus far have a variety of optical problems including low power handling capability, low reflectivity, high scatter loss, and poor adjacent channel crosstalk.